In many cases mechanical springs are used in railway car suspension systems, sometimes accompanied by bellows air spring. Mechanical springs generally cause vibrations, surges, and noise which are generally transmitted from the truck to the car body. This causes passenger discomfort. The vibrations, surges, and noise result from a number of operating conditions. For example, the mechanical springs may have natural frequencies at which they tend to vibrate. Sudden impacts transmitted from the tracks through the wheels of the truck may cause surges and noise in the springs which in turn are transmitted to the car body.
Different types of damping elements have been used in connection with mechanical springs. Generally, these have been located at the top and bottom of the springs. Such arrangements have not been entirely successful in sufficiently damping the vibrations and preventing them from being transmr,tted to the railway car body.
Recently there have been developed by Firestone, a so-called "Marsh Mellow".RTM. spring, which eliminate many problems relating to vibrations. One example of these springs include an elastomeric core with a hollow center with several plies of cord-reinforced fabric serving as the outer cover.
In railway car applications, it was found that it is necessary to provide springs for a suspension system which control the relative spring rates between the vertical and lateral movements. Very often, it is desirable to provide a predetermined vertical spring rate with restrictive lateral spring rates. Vertical spring rates in a single spring are limited because making a single spring too high creates a tendency of the spring to tilt over.
The main patents found during a search of the subject matter of this invention included U.S. Pat. Nos. 4,080,061; 3,910,655; 3,799,066; and 4,174,140. None of these patents disclosed a pair of stacked springs with one of the springs being elastomeric and one being restrictive in lateral movement.